<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/vue.js"></script>
</head>

<body>
    <div id="demo">
        <p>天气： {{weather}}</p>
        <button @click="isHot=!isHot">变天</button>
    </div>
    <script>
        new Vue({
            el: '#demo',
            data: {
                isHot: true,
                weather: ''
            },
            watch: {
                isHot: {
                    
                    /**
                     * 默认首次挂载不执行
                     * immediate 立即 马上
                     */
                    immediate:true,
                    handler(value) {
                        this.weather = this.isHot ? '炎热' : '凉爽'
                    }
                }

            }
        })
    </script>
</body>

</html>